We have undertaken two projects aimed at characterization of the genus of P. berghei and S. mansoni. To study the malarial parasite, P. berghei, we are developing a series of polymorphic genetic DNA markers using DNA recombinant technology. These markers will prove useful not only in the study of malarial genetics but also as tools that can be used to distinguish closely related lines of the parasite. To date we have cloned over 1000 individual DNA segments from the P. berghei genome in the E. coli plasmid vector pBr322. We have radioactively labeled some of these recombinant plasmids and have used them as probes in DNA studies using the Southern Blot technique. We have found that 4 of the recombinant plasmids studied give easily recognized patterns of bands when hybridized to genomic DNA from P. berghei. This will make them useful as probes for DNA polymorphism. We have found that the pattern of DNA hybridization varies when genomic DNA from different species of mouse malarial parasites are tested and we take this as an indication that these markers will also be useful in population studies. We have undertaken the S. mansoni project in collaboration with A. Simpson and A. Sher. We are studying the reassociation kinetics in solution of